Nuclear magnetic resonance analysis of multiple samples

ABSTRACT

A Nuclear Magnetic Resonance (NMR) probe device (20) is disclosed. NMR probe device (20) includes a plurality of detection coils (30, 40) each operable to detect a signal from a corresponding one of a plurality of samples (34, 44) undergoing NMR analysis. Also included is a plurality of tuning circuits (31, 41, 38, 48) each coupled to one of detection coils (30, 40) to tune the one of the detection coils (30, 40) to a corresponding resonant frequency for the NMR analysis of the corresponding one of the samples. An electromagnetic shield (22) is positioned between a first one of the detection coils (30, 40) and a second one of the detection coils (30, 40) to isolate the first one of the detection coils (30, 40) and the second one of the detection coils (30, 40) from each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication No. 60/121,869, filed Feb. 26, 1999, which is herebyincorporated by reference in its entirety; and is a continuation ofInternational Patent Application No. PCT/US00/04842 filed Feb. 25, 2000and published in English Aug. 31, 2000.

GOVERNMENT RIGHTS

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of the NationalScience Foundation (NSF) grant number CHE 95-31693 under contract number500-1393-3104.

BACKGROUND

The present invention relates to analysis of materials based on NuclearMagnetic Resonance (NMR), and more particularly, but not exclusively,the NMR analysis of multiple samples.

Atomic nuclei with an odd atomic mass or an odd atomic number possess anuclear magnetic moment. NMR methods are based on the absorption andre-emission of radio frequency waves by a sample in a magnetic fieldthat have atoms with this nuclear make-up. By way of nonlimitingexample, molecules including ¹H, ¹³C, ¹⁹F, or ³¹P may be analyzed usingNMR techniques to provide fast, molecule-specific qualitative andquantitative information. Such molecules exhibit resonant frequenciesthat are sensitive to the molecular chemical environment, making NMR auseful molecular probe. However, existing NMR equipment is generallyunable to satisfactorily analyze more than one sample at a time, andcorrespondingly limits sample evaluation throughput. To provide for moreefficient use of NMR resources, techniques to increase sample throughputwould be desirable.

SUMMARY

One form of the present invention includes a unique system to performNMR evaluation on more than one sample at a time. Alternatively oradditionally, another form of the present invention includes a uniquetechnique to perform NMR analysis of multiple samples simultaneously.

A further form of the present invention includes NMR instrumentationcomprising a probe with a plurality of coils each configured to receivea different sample. The coils each include a tuning circuit. Theseturning circuits may be located external to the NMR magnet and may beelectrically shielded from one another to reduce unwanted interactions.

In another form, a technique of the present invention includes providingan NMR probe with multiple coils each arranged to receive acorresponding one of a plurality of samples. The coils may each be tunedseparate from the others to provide for simultaneous evaluation of thesamples. The coils may each be coupled to a tuning circuit having avariable element that is adjusted to perform the tuning operation. Foreach tuning circuit, the coil may be coupled to the variable element bya transmission line to provide for remotely locating the variableelement outside of the NMR magnetic field while the coil remains in thisfield. In one embodiment, the tuning circuit includes two variablecapacitor elements remotely located relative to the coil by atransmission line coupling, and another two fixed or variable capacitorscoupled either in parallel or series, or both, in the NMR magnet beforethe transmission line coupling to the tuning circuit.

In yet another form, an NMR probe is provided that includes a number ofcoils each configured to receive a different sample. This probe may beincorporated into standard NMR equipment with only minor modificationsto facilitate the simultaneous detection of multiple samples. Forexample, the probe may be arranged to fit into a conventional NMR magnethousing and use a common NMR transmitter amplifier for excitation ofmultiple samples.

Other forms of the present invention include a multicoil NMR probearranged for operation in a magnetic field with a predeterminedgradient. This gradient is used to differentiate multiple samples. Thedata from multiple coils in the graded field may be received by a singlereceiver and analyzed using one or more procedures to provide thedesired differentiation. In one embodiment, a two dimensionalrepresentation of the data is created to better differentiate thesamples.

Further forms, embodiments, advantages, benefits, aspects, and objectsof the present invention shall become apparent from the description andfigures provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial schematic view of a NMR system of one embodiment ofthe present invention.

FIG. 2 is a partial sectional and schematic view of a probe device forthe system of FIG. 1.

FIGS. 3 and 4 are diagrams of proton coupled ¹³C NMR spectra obtainedwith the system of FIG. 1 for methanol and acetone, respectively. Bothspectra were acquired using a single 90 degree pulse and the same 50 kHzspectral widths. J couplings evident in the spectra are 138 Hz formethanol and 140 Hz for acetone.

FIG. 5 depicts ¹³C NMR spectra for methanol and carbon tetrachlorideusing the system of FIG. 1.

FIG. 6 is a partial schematic view of a NMR system of another embodimentof the present invention.

FIG. 7 is a partial schematic view of a probe device for the system ofFIG. 6.

FIG. 8 is a partial sectional and schematic view of the probe device forthe system of FIG. 6.

FIG. 9 is a flow chart of a process for operating the system of FIG. 6.

FIG. 10 is a diagram illustrating a composite NMR spectrum of multipleH₂O/D₂O samples obtained with the probe device of FIG. 6 in asubstantially homogeneous magnetic field.

FIG. 11 is a diagram illustrating a composite NMR spectrum of the sameFIG. 10 H₂O/D₂O samples obtained with the probe device of FIG. 6 in agraded magnetic field.

FIG. 12 depicts a first composite ¹H NMR spectrum for 0.5 M samples ofmethanol, acetonitrile, t-butanol, and water in a substantiallyhomogeneous magnetic field, and a second composite ¹H NMR spectrum ofthese samples with a magnetic field gradient applied.

FIG. 13 depicts multiplication of the two spectra in FIG. 12 as afunction of the frequency offset to determine sample region frequencyshifts.

FIG. 14 depicts separated ¹H NMR spectra of each of the samples of FIG.12.

FIG. 15 is a flow chart of one procedure for determining sample-specificspectra for the process of FIG. 9.

FIG. 16 depicts a composite ¹H NMR spectrum for samples of H₂O,methanol, t-butanol and acetonitrile (all 500 MM in D₂O) acquired withthe system FIG. 6 at 300 MHz.

FIG. 17 depicts a series of ¹H NMR spectra recorded with different Z1shim values to provide different corresponding gradient strengths of 0mG cm⁻¹, 12 mG cm⁻¹, 24 mG cm⁻¹, and 48 mG cm⁻¹, respectively, for thesamples of FIG. 16.

FIG. 18 is a flow chart of another procedure for determiningsample-specific spectra for the process of FIG. 9.

FIGS. 19-22 depict selected stages of the procedure of FIG. 18 asapplied to the samples of FIG. 17.

FIG. 23 depicts the differentiated sample spectra obtained with theprocedure of FIG. 18.

FIG. 24 depicts comparative stages of reference deconvolution for amethanol spectral peak.

FIG. 25 depicts a 2D COSY spectrum of samples of 0.50 M ethanol,1-propanol, dichloroacetic acid, and acetaldehyde in D₂O without amagnetic field gradient.

FIG. 26 depicts the 2D COSY spectrum of FIG. 25 overlaid with a gradientshifted spectrum.

FIG. 27 depicts a separated 2D COSY sub-spectrum for 1-propanolgenerated from the gradient shifted and unshifted spectra of FIG. 26.

FIG. 28 depicts a separated 2D COSY sub-spectrum for ethanol generatedfrom the gradient shifted and unshifted spectra of FIG. 26.

FIG. 29 is a flow chart of a further process for operating the system ofFIG. 6.

FIG. 30 is a diagram of spectral results illustrative of the process ofFIG. 29.

FIG. 31 is a partial schematic view of a NMR system of a furtherembodiment of the present invention.

FIG. 32 is a diagram illustrating selected operating characteristics ofthe NMR system shown in FIGS. 31 and 33.

FIG. 33 is a partial schematic view of a NMR system of yet a furtherembodiment of the present invention.

FIG. 34 is a partial diagrammatic view of an NMR system of still afurther embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any alterations and further modificationsin the described embodiments, and any further applications of theprinciples of the invention as described herein are contemplated aswould normally occur to one skilled in the art to which the inventionrelates.

FIG. 1 schematically illustrates Nuclear Magnetic Resonance (NMR) system10 of one embodiment of the present invention. Instrumentation of system10 includes Radio Frequency (RF) transmitter 12 coupled to powersplitter 13. Power splitter 13 has two outputs 13 a, 13 b coupled toduplexers 14 a, 14 b, respectively. Each duplexer 14 a, 14 b is operablycoupled to a corresponding probe channel CH1, CH2. Duplexers 14 a, 14 bpass high-power level RF signals from RF transmitter 12 to probe device20 via channels CH1, CH2.

Probe device 20 is removably positioned in sample space 15 of NMR magnet16. Probe device 20 is configured to place two samples in sample space15 for simultaneous NMR analysis. Referring additionally to FIG. 2,probe device 20 includes housing 20 a that defines probe head 50opposite base 60. Two detection coils 30, 40 are disposed within probehead 50. Coils 30, 40 are each disposed about a corresponding sampleholder 30 a, 40 a. Sample holders 30 a, 40 a are each arranged to hold adifferent sample, and maintain the samples spatially separated from oneanother within probe head 50. In one embodiment, coils 30,40 are eachprovided in the form of a helical winding (alternatively designated a“solenoid” configuration herein) about a glass tube which serves as thecorresponding sample holder 30 a, 40 a. In other embodiments, coil 30and/or coil 40 can be of a different type, including, but not limited toa saddle or bird cage coil geometry, and holders 30 a, 40 a can beconfigured for another container type and/or composition. U.S. Pat. No.4,654,592 to Zens; U.S. Pat. No. 5,323,113 to Cory, et al.; and U.S.Pat. No. 5,929,639 to Doty provide a few nonlimiting illustrations ofvarious types of coil geometry.

Probe device 20 includes channel circuitry 21 to independently coupleeach coil 30, 40 to a different connector 23 a, 23 b in base 60 incorrespondence with probe channels CH1, CH2, respectively. In otherembodiments, probe device 20 includes additional coils withcorresponding channel circuitry and connectors to provide more than twoprobe channels. Accordingly, for such embodiments, more than two samplescan be submitted for simultaneous NMR analysis.

Probe device 20 further includes grounding plane 22 coupled to housing20 a and channel circuitry 21. Grounding plane 22 is positioned betweencoils 30,40 to reduce intercoil cross-talk. Grounding plane 22 is in theform of a plate comprised of copper that has a thickness suitable tooperate as shielding to electromagnetically decouple coils 30,40 fromeach other. In other embodiments, grounding plane 22 may be of adifferent form or composition, and/or an electromagnetic shield may beprovided in a different manner. In still other embodiments, groundingplane 22 and/or an electromagnetic shield may be absent.

Circuitry 21 includes fixed tuning networks 31, 41 disposed within probehead 50. Fixed tuning networks 31, 41 are electrically coupled to coils30, 40, respectively, and each belong to a different probe channel CH1,CH2. Fixed tuning networks 31, 41 include capacitive elements 32, 42electrically connected in parallel with coils 30, 40, respectively.Tuning network 31 includes capacitive element 34 electrically connectedin series with the parallel circuit coil 30 and capacitive element 32.Tuning network 41 includes capacitive element 44 electrically connectedin series with the parallel circuit of coil 40 and capacitive element42.

Circuitry 21 also includes coaxial transmission lines 36, 46 andadjustable tuning networks 38, 48. Fixed tuning networks 31, 41 areelectrically connected to the inner conductor of coaxial transmissionlines 36, 46, respectively. Transmission lines 36, 46 interconnect fixedtuning networks 31, 41 in probe head 50 with adjustable tuning networks38, 48 disposed in base 60 of probe device 20. Adjustable tuningnetworks 38, 48 include adjustable capacitive elements 38 a, 48 aelectrically connected between the inner conductor termination oftransmission line 36, 46 in base 60 and ground. Adjustable tuningnetworks 38, 48 also include adjustable capacitive elements 38 b, 48 bbetween connectors 23 a, 23 b and the termination of the inner conductorof transmission lines 36, 46, respectively.

System 10 further includes preamps 17 a, 17 b; receivers 18 a, 18 b; andreference frequency source 19. Probe channels CH1, CH2 are eachelectrically connected to a corresponding preamp 17 a, 17 b and NMRreceiver 18 a, 18 b. Receivers 18 a, 18 b are electrically coupled toreference frequency source 19 in a standard manner. Fixed tuningnetworks 31, 41 and adjustable tuning networks 38, 48 are arranged totune to a resonant frequency for NMR analysis of a nucleus type commonto each of the samples held in coils 30, 40. During exposure to themagnetic field generated by NMR magnet 16, a suitable RF signal fromtransmitter 12 delivered to each coil 30, 40 excites the correspondingsamples. Duplexers 14 a, 14 b are arranged to route the RF excitation tochannels CH1, CH2 through crossed diode pair DP1 while preamps 17 a, 17b are blanked to present a high input impedance. Crossed diode pair DP2associated with each channel CH1, CH2 provides further circuit isolationand protection. Typically, RF excitation is in the form of a common 90degree pulse; however, other interrogation techniques may alternativelyor additionally be utilized as would occur to those skilled in the art.

After excitation, coils 30, 40 are also used to detect a response forthe sample contained in its corresponding sample holder 30 a, 40 a. Thisresponse is provided by each coil 30, 40 as an electrical signal alongthe corresponding channel CH1, CH2. Preamps 17 a, 17 b are activated toreceive the response signals from channels CH1, CH2 via duplexers 14 a,14 b for processing by NMR receivers 18 a, 18 b in the usual manner.

Placement of adjustable tuning networks 38, 48 in base 60 provides foreasy accessibility and a reduction in the volume and complexity of probehead 50. The separate tuning networks 38, 48 allow each coil 30, 40 tobe tuned independently to the desired resonant frequency. Typically, foreach channel CH1 and CH2, this resonant frequency is selected tointerrogate the same nucleus type. Fixed networks 31, 41 provide coarsetuning and reduce power losses that would otherwise occur intransmission lines 36, 46. Fine adjustment of the tuning and matching ofprobe circuit is accomplished by adjusting the pair of tunablecapacitors 38 a, 48 a and 38 b, 48 b in each corresponding network 38,48. Isolation is provided by shielding of the individual coils and byappropriate grounding. In particular, ground plane 22 separates the twodetection coils 30, 40 in the sample region (probe head 50), whileseparate compartments house each network 38, 48 at probe base 60. As aresult, each circuit tunes independently and cross-talk between coils30, 40 is reduced.

Coils 30, 40 each extend along a corresponding longitudinal axis 30 b,40 b as illustrated in FIG. 2. Axes 30 b, 40 b are approximatelyparallel to each other. Alternatively or additionally, cross-talkreduction can be obtained by orienting one of the coils 30, 40 relativeto another of the coils 30, 40 based on coil geometry. By way ofnonlimiting example, by orienting the longitudinal axes 30 a, 40 a ofthe coils 30, 40 to cross one another at approximately right angles (90degrees), cross-talk can generally be reduced for a saddle, solenoid, orother generally cylindrical coil configuration.

The experimentally observed spectra depicted in FIGS. 3 and 4 illustratethe type of data that may be acquired with system 10. For thisexperimental example, two coils were provided in the form of a 4 turninductor of solenoid geometry wrapped from 20 gauge insulated magnetwire that were each attached to a glass tube using a common epoxyadhesive. This glass tube was about 30 mm long with about a 4 millimeter(mm) outer diameter (o.d.) and about a 2 mm inner diameter (i.d.). Eachsample was placed in a sealed glass tube having about a 2 mm innerdiameter and a length of about 6 mm that was positioned into acorresponding one of the larger glass tubes. A nominal 11 picofarad (pf)fixed capacitor was used for each of the parallel capacitive elements32, 42 (American Technical Ceramics Corp., Huntington Station, N.Y.).For this embodiment, a nominal 32 picofarad (pf) capacitor was utilizedfor each element 34, 44 between the network 31, 41 and the transmissionline 36, 46, respectively; and tuning elements 38 a, 38 b, 48 a, 48 b ofnetworks 38, 48 were provided in the form of capacitors having anominally variable range of about 3 pf to about 11 pf (VoltronicsCorporation, Denville, N.J.). The regions of the probe base containingthe variable capacitors were electronically isolated from one another.It was observed that the coils did not exhibit coupling due to mutualinductance when tuned and matched to the same resonant frequency;thereby allowing different samples to be monitored using a single NMRspectrometer.

For FIGS. 3 and 4, spectral data was acquired at about 7.4 Tesla andboth spectra were acquired at the same time in response to a single 90degree RF pulse from the RF transmitter operating at a frequency of75.44 MHz (corresponding to 75.440 MHz for ¹³C). In this example, thetwo spectra were acquired on two separate NMR receivers. The transmittedRF excitation pulse was split through the power splitter (ModelZSC-2-1W, Mini-Circuits, Brooklyn N.Y.), and each output from the powersplitter was routed through crossed diodes (to reduce amplifier noise)and subsequent, independent duplexer/preamp stages. The crossed diodepairs DP1, DP2 also improved signal isolation, reducing interferenceduring data acquisition. The receivers were phased locked using a 10 MHzreference signal from the Varian spectrometer. Simultaneous dataacquisition was accomplished by using Analog-to-Digital (A/D)conversion.

In FIGS. 3 and 4, ¹³C NMR spectra of the two different samples eachrepresenting a single analyte compound were acquired at the same time inseparate, discrete detection coils. In one coil, a sample of about 4 μlof acetone, isotopically enriched to 99% at the methyl position wasdetected (FIG. 4). In the other coil, a sample of ¹³C enriched methanol(also 99%) of similar size was detected (FIG. 3). Enriched compoundswere used to enhance the signal to noise ratio for preliminaryinvestigations, however, the observed mass-limited sensitivity wasbetter than that typically achieved in standard NMR probes and is inline with the sensitivity of small microcoils in general. Both spectraof FIGS. 3 and 4 were acquired with the same 50 kHz spectral widths. Jcouplings evident in the spectra are 138 Hz for methanol and 140 Hz foracetone.

Referring next to the experimental example corresponding to the spectraof FIG. 5, the same experimental set-up was used as that described forthe spectra of FIGS. 3 and 4. In FIG. 5, 4 μl samples of methanol (¹³C,99%) and carbon tetrachloride (¹³C, 99%) were evaluated corresponding tospectra 70, 80. Each spectrum 70, 80 is the result of a singleacquisition using a 10 μs, 90 degree pulse and a transmitter power ofabout 3 watts measured at the output of RF transmitter 12. Spectrum 70shows the proton spin-coupled ¹³C-methanol quartet at 50.3 ppm relativeto TMS. The J-coupling is 141 Hz and the linewidth is approximately 9.2Hz Full Width Half Maximum (FWHM). Spectrum 80 shows a ¹³C singlet fromcarbon tetrachloride at 96 ppm relative to TMS, with a line width ofapproximately 10.7 Hz (FWHM). No evidence of cross talk was present inthe spectra, even after signal averaging (100 averages, not shown) wasperformed, to better than 1 percent of the signal intensity.

In an alternative embodiment of system 10, a single receiver may beutilized that is switched between channels CH1 and CH2 to acquire data.Correspondingly, a single computer can be used to operate thespectrometer and to acquire the signals from different samples by usinga multichannel analog to digital (A/D) converter. The selectedfrequencies of the transmitter and receiver may vary according theparticular application as would occur to those skilled in the art.Further, the RF transmitter, duplexers, receivers, and associatedconnections of system 10 may otherwise be arranged as would occur tothose skilled in the art without departing from the spirit of thepresent invention. In other embodiments, other coil types and geometriesmay be utilized, and/or the coils may be differently sized inalternative embodiments, including the microcoil variety. As usedherein, a “microcoil” has a maximum diameter of no more than about 1millimeter (mm). In still other embodiments, more than two samples maybe evaluated simultaneously in the same NMR probe by adding coilssuitably decoupled by appropriate ground planes, shielding, and/or coilorientations, with corresponding tuning circuitry, duplexers, associatedconnections, and (optionally) receivers in accordance with the teachingsof the present invention. Alternatively or additionally, RF excitationmay be provided to each sample from a different source than thedetection coil, such as a dedicated excitation coil.

FIG. 6 illustrates system 110 of another embodiment of the presentinvention. For system 110, multiple samples are differentiated byapplying a magnetic field gradient. When a magnetic field with agradient component is applied, samples in different regions of spaceexperience different magnetic fields. A spatially dependent frequencyoffset is introduced by the magnetic field gradient that is unique toeach properly positioned sample. The application of field gradientsallows for the signals from multiple samples to be detected using only asingle receiver. Accordingly, differentiation of spectra for multiplesamples can be determined from a two dimensional representation, withthe first dimension providing the spatial information and the seconddimension providing spectral information. By combining this technologywith NMR microcoils, a substantial number of samples, limited only bythe usable region of the NMR magnetic field, can be simultaneouslyanalyzed.

System 110 includes NMR spectrometer instrumentation 111 operativelycoupled to processor 119 and removable probe device 120. As depicted inFIG. 6, probe device 120 is disposed in sample space 115 of NMR magneticfield source device 116. Furthermore, probe device 120 is coupled tosample control instrumentation 122. NMR spectrometer instrumentation 111includes a controllable RF transmitter 112 and NMR receiver 118 commonlycoupled to probe device 120 by probe channel PC1. NMR spectrometerinstrumentation 111 also includes controller 113 to control theoperations of RF transmitter 112 and receiver 118 and to provide aninterface with processor 119.

processor 119 and the constituents of in instrumentation 111 (such astransmitter 112, controller 113, and/or receiver 118) may be comprisedof one or more components integrated to automatically process a numberof samples. Alternatively, one or more components of system 110 may beremotely located relative to the others, and/or may be configured tooptionally provide remote control of NMR processing with system 110. Inone embodiment, processor 119 is in the form of a desktop PersonalComputer (PC) programmed to perform the various indicated operations,and includes various input devices, such as a keyboard and/or mouse; andvarious output devices, such as a graphic display, printer, and/orplotter. For this embodiment, instrumentation 111 is in the form of astandard NMR spectrometer that provides spectral data to processor 119by portable disk and/or a hardwired interface.

Instrumentation 111 also includes magnetic field adjustment control 117.Control 117 is operatively coupled to magnetic field source 116 toregulate homogeneity of the magnetic field generated in sample space 115and selectively introduce one or more field gradients along a selecteddirection and/or of a selected magnitude in sample space 115. Control117 can be an integral part instrumentation 111, can be in the form ofone or more separate operator-adjustable devices attached to magneticfield source device 116, a combination of these, or such otherarrangement as would occur to those skilled in the art.

Referring additionally to FIG. 7, further details of probe device 120are illustrated. Probe device 120 includes probe circuitry 121 with fourexcitation/detection coils 130 a, 130 b, 130 c, 130 d (collectivelydesignated detection coils 130) electrically connected in parallel. Eachcoil 130 is of a solenoid geometry. In one embodiment, one or more ofcoils 130 are formed using the techniques described in D. L. Olson, T.L. Peck, A. G. Webb, R. L. Magin and J. V. Sweedler, Science, 270 (1995)1967, and A. G. Webb and S. C. Grant, J. Magn. Reson.B, 113 (1996) 83,which are hereby incorporated by reference. In other embodiments, adifferent coil geometry such as a saddle or bird cage type, may beutilized for one or more of coils 130. Each coil 130 a, 130 b, 130 c,130 d is disposed about a sample holder 132 a, 132 b, 132 c, 132 d(collectively designated sample holders 132), respectively. Each sampleholder 132 is in the form of a tube that is open at opposing ends andcarries the corresponding coil 130.

Referring also to FIG. 8, sample holders 132 are mounted in U-shapedyoke 152 connected to probe base 160. The opposing ends of sampleholders 132 are each in fluid communication with a pair of sampleconduits 136 a, 136 b (collectively designated sample conduits 136) thatare connected to sample control instrumentation 122. In cooperation withsample control instrumentation 122, sample holders 132 a, 132 b, 132 c,132 d are each arranged to selectively receive a corresponding fluentsample 134 a, 134 b, 134 c, 134 d (collectively designated samples 134).Samples 134 may be changed from time-to-time through the correspondingpair of conduits 136 a, 136 b without removing probe device 120 fromsample space 115. Accordingly, the degree of likelihood that adjustmentswill need to be made between sample interrogations is reduced,potentially increasing throughput. Also, a higher throughput may berealized compared to standard NMR equipment that only tests one sampleat a time. This sample delivery and exchange arrangement is alsosuitable for performing various complex molecular analysis techniques,including but not limited to Capillary Electrophoresis (CE), and LiquidChromotography (LC) NMR (LC-NMR).

Probe circuitry 121 includes tuning network 140 with adjustablecapacitor elements 142 and 144. Adjustable capacitor element 142 iselectrically connected in parallel with coils 130. Capacitor element 144is electrically connected between the parallel circuit of coils 130 andcapacitor element 142, and the inner conductor of coaxial transmissionline 146. The outer shield conductor of transmission line 146 isconnected to ground along with a node common to coils 130 and capacitorelement 142. As shown in FIG. 8, the opposite end of transmission line146 terminates in a BNC connector 123 to couple with probe channel PC1(see FIG. 6). The interconnection of tuning network 140 to coils 130 isnot illustrated in FIG. 8 to preserve clarity.

Tuning network 140 provides for tuning to a resonance frequencyappropriate for NMR interrogation of samples 134 and is located inadjacent probe base 160 at the bottom of yoke 152. Probe base 160 isthreaded to receive a threaded housing 150. When threaded together,probe base 160 and housing 150 define fluid chamber 166. Chamber 166 isarranged to optionally receive and contain a susceptibility matchingfluid to further improve resolution of spectra detected with coils 130.

The flow chart of FIG. 9 depicts representative stages of process 210 toperform NMR analysis of samples 134 with system 110. Process 210 startswith stage 212. In stage 212, sample control instrumentation 122 isutilized to load samples 134 into holders 132 via conduits 136 a, andflush-out any previous samples via conduits 136 b. After samples 134 areloaded into holders 132, a generally homogeneous magnetic field isapplied in sample space 115 with magnetic source device 116 and samples134 are excited with an appropriate RF signal from transmitter 112 viacoils 130 in stage 214. During stage 214, the four regions correspondingto coils 130, holders 132, and samples 134 experience generally the samemagnetic field, B_(o). Process 210 continues with stage 216. In stage216, the collective response of samples 134 to the RF excitation signalsare detected with coils 130 and transmitted as response signals by coils130 to receiver 118 for analysis. The corresponding spectral data S₀ ofsamples 134 is determined from the coil response signals and stored byinstrumentation 111 and/or processor 119. Referring additionally to FIG.10, a composite spectrum for an identical sample 134 of H₂O/D₂O in eachcoil 132 of probe device 120 is illustrated. This composite spectrum hasa single peak corresponding to a single resonance in the homogenousfield.

In stage 218, a magnetic field gradient component is applied along avertical axis z (see FIG. 8) that traverses samples 134 when disposed insample space 115. When the field gradient is applied, each of the sampleregions experience a field given by B_(o)+G_(z)×z_(i), where G_(z) isthe strength of the linear field gradient and z_(i) is the verticalposition of the i-th sample. The positions z₁, z₂, z₃, z₄, of Coils 130a, 130 b, 130 c, and 130 d along the z axis are illustrated in FIG. 8with respect to the longitudinal centerline axis 133 a, 133 b, 133 c,133 d of coils 130 a, 130 b, 130 c, 130 d. Correspondingly, magneticfield strength across samples 134 changes with position along the zdirection and causes different amounts of frequency shift in theindividual responses of samples 134. In stage 220, the frequency-shiftedspectral data S_(G) is determined from corresponding coil responsesignals and stored by instrumentation 111 and/or processor 119.

Referring also to FIG. 11, a collective spectrum of the same samplesthat were the subject of the spectrum of FIG. 10 are shown; however, forFIG. 11 the applied field gradient shifts the resonance frequency ofeach sample 134 by a different amount relative to the single resonanceof FIG. 10. Accordingly four different peaks are observed in FIG. 11. Togather the data for the spectra of FIGS. 10 and 11, probe device 20 wascentered in device 116 by loading four H₂O/D₂O samples and adjusting thelinear gradient to separate the peaks from the individual coils. Thecenter was chosen as the point at which the top and bottom coils wereshifted in frequency by an equal and opposite amount.

Process 210 resumes with stage 230. In stage 230, the data for S₀ andS_(G) is analyzed to differentiate the spectra of each sample 134 fromone another using one or more various techniques, a few examples ofwhich are as follows. Stage 230 techniques typically utilize frequencyshift values associated with each coil 130/sample 134. Theses values maybe determined in various ways and typically are partially or completelyexecuted by programming of processor 119 using spectral data obtainedfrom instrumentation 111. One procedure to determine theposition-dependent frequency shifts begins with the acquisition of twospectra for a set of identical analytes having a relatively simpleresonance pattern, such as the H₂O/D₂O samples. The first spectrum isobtained under generally homogeneous magnetic field conditions as in thecase of FIG. 10; and a second spectrum is obtained while applying areproducible, known field gradient to the same samples as in the case ofFIG. 11. The degree of frequency shift of the resonance pattern for eachsample can be measured by comparing the peak patterns of the twospectra, and a corresponding set of sample region frequency shift valuescan be prepared for application to different samples under the samegraded and ungraded magnetic field conditions.

One procedure to determine the frequency shift experienced by each coil130 during the application of the gradient is through comparativemeasurements with an external standard such as water, loaded in allcoils 132 under both differently graded magnetic field conditions. Onoccasion, this external calibration may result in small differences inlocal electromagnetic fields that occur because of susceptibilitychanges with different solvents or upon reloading probe device 120 intosample space 115. In applications where it is desirable to reduce suchdifferences, another procedure determines frequency shifts directly fromthe original spectrum, S₀, and the frequency-shifted gradient spectrum,S_(G), undergoing analysis. In this case, the shifts can be determinedby calculating the overlap via multiplication of S₀ and S_(G) inaccordance with equation (1) as follows: $\begin{matrix}{{S_{overlap}\left( \Delta_{i} \right)} = {\sum\limits_{N}{{S_{0}(\omega)} \times {S_{G}\left( {\omega + \Delta_{i}} \right)}}}} & (1)\end{matrix}$

where Δ_(k), is the frequency shift for an individual coil k, and N isthe number of points in the spectrum.

Referring to FIG. 12, an experimental NMR ¹H spectrum 230 a isillustrated for 0.50 M samples of water (δ=4.7 ppm), acetonitrile (2.0ppm), methanol (3.3 and 4.73 ppm) and t-butanol (1.2 and 4.76 ppm) inD₂O in separate coils of a four-coil probe. The spectrum of FIG. 12 wasobtained without a predetermined magnetic field gradient in sample space115 to correspond to spectrum S₀ obtained in stage 216. FIG. 12 alsoincludes a frequency-shifted spectrum 230 b for the same sample set asused to provide spectrum 230 a. For spectrum 230 b, an applied gradientof about 48 mG/cm was provided to correspond to spectrum S_(G) obtainedin stage 220. Each of the peaks in spectrum 230 b shifts a differentamount or in a different direction according to the position of eachindividual coil in the applied field gradient. The OH region is splitinto four lines due to contributions to this peak from each of thecoils.

Referring to FIG. 13, an overlap plot obtained by applying equation (1)to the data for spectra 230 a, 230 b of FIG. 12 is illustrated. Theoverlap plot of FIG. 13 shows four maxima at the frequency shiftscorresponding to those for the four coils. The overlap intensity isproportional to the square of the signal amplitude in each coil. Theshift values obtained using this method match well with those derivedfrom the four H₂O/D₂O samples described in connection with FIGS. 10 and11, but require only two measurements (original and shifted spectra) asopposed to four for external referencing.

Once the frequency-shifts of the coils have been measured, severalapproaches can be implemented to assign the peaks. One technique whichcan be readily executed by processor 119 is to shift the gradientspectrum S_(G) and take its product with the original spectrum S₀ inaccordance with equation (2) as follows:

 S ^(i)(ω)=S ₀(ω)×S _(G)(ω+Δ_(i))  (2)

where, S^(i) is the spectrum generated for the ith coil and Δ_(i) is itscorresponding frequency shift. The results of this multiplicationprocedure on the S₀ and S_(G) spectra of FIG. 12 are shown in FIG. 14.In FIG. 14, spectrum 230 a is duplicated for comparison. Also, thesub-spectra 231 a, 231 b, 231 c, 231 d are illustrated that correspondto the different water, methanol, acetonitrile, and t-butanol samples.Accurate quantitative information can be restored for this technique bytaking the square of the product; however, this approach may introduceartifacts when multiplying noisy regions in the two spectra. Forapplications where it is desirable to reduce this possibility, thenanother differentiation technique may be utilized as appropriate.

The flow chart of FIG. 15 depicts one of the other techniques asprocedure 310. As in the case of the shift identification proceduredescribed in connection with FIGS. 12 and 13, and the multiplicationtechnique described in connection with FIG. 14; processor 119 may besuitably programmed to execute procedure 310. Procedure 310 begins withstage 312. In stage 312, a counter PC is set to 1 (PC=1). Counter PCcorresponds to a number of identified peaks determined in stage 314. Acoil counter CC is also set to 1 (CC=1), that corresponds to the numberof detection coils 130. Next, procedure 310 resumes with stage 314 thatgenerates a table of the peaks versus frequency for the S₀ and S_(G)spectra, then processing loop is entered beginning with stage 316. Instage 316, the spectral peaks of S_(G) are shifted by the previouslydetermined frequency shift for the currently indexed coil CC.Conditional 318 tests whether the indexed peak PC of the S_(G) spectrumsuitably matches a peak of the S₀ spectrum. If there is a tentativematch, an entry is made in an output peak table in stage 320 for thegiven indexed coil CC. Procedure 310 resumes with conditional 322. Ifthere is no match as determined with conditional 318, then process 310proceeds directly from the negative branch of conditional 318 toconditional 322.

Conditional 322 tests if the indexed peak PC is the last identified peakof the S_(G) spectrum to be analyzed. If the test of conditional 322 isnegative, then procedure 310 continues with stage 324 to increment thepeak counter PC to the next peak (PC=PC+1). From stage 324, process 310loops back to execute conditional 318 for this next peak.

If the test of conditional 322 is affirmative, conditional 326 isencountered, which tests whether coil CC is the last coil. If coil CC isthe last coil (CC=LAST), then procedure 310 proceeds to stage 330 toprovide the spectra of the samples 134 is terms of the various entriesin the output table grouped together for each different coil 130. Stage330 may also include logic to resolve any detected ambiguities orunmatched peaks of the S₀ and/or S_(G) spectra. If coil CC is not thelast coil as tested by conditional 326, process 310 continues with stage328. In stage 328, the peak counter PC is reset to 1 (PC=1) and the coilcounter CC is incremented to point to the next coil (CC=CC+1). Procedure310 then returns to stage 316 to shift the S_(G) peaks with thefrequency shift for this next coil. Stages 316, 320, 326, 328 andconditionals 318, 322, 324 are then repeated as the various conditionaltests dictate until all peaks of the S_(G) spectrum have been consideredwith the frequency shift of each detection coil 130. Once all thesepeaks are analyzed, then the previously described stage 330 isencountered to provide peak tables for each coil 130. Procedure 310 thenterminates.

Experimental verification of procedure 310 was conducted. For thisexperimental example, four coils were wrapped from a high puritypolyurethane coated 36-gauge copper wire (California Fine Wire Co.Grover Beach, Calif.) around fused silica capillaries (about 20 mm long,about a 1.6 mm outer diameter, about a 0.8 mm inner diameter) whichserved as both the coil form and sample holder. The inductance of eachcoil was approximately 20 nanohenries (nH). The coils were attached tothe capillary tubes using a cyanoacrylate adhesive (Krazy Glue, BordenInc. Columbus, Ohio). The coils were configured with four (4) turns eachhaving an inner diameter of about 1.6 mm and a length of about 1.0 mm.The sample tubes were mounted in a PVC coil holder (corresponding toyoke 152) that held the capillary tubes with an intercoil spacing(center to center) of about 3.2 mm. The entire coil array was housed ina removable PVC container that was filled with Fluorinert FC-43 (SynQuest Laboratories, Alachua, Fla.), a susceptibility matching fluid thathas been shown to improve magnetic field homogeneity by minimizing fielddistortions induced by copper NMR coils. The PVC container was threadedand employed o-ring seals to prevent leakage of the Fluorinert fluid.The coil leads were connected in parallel and cut to the same length sothat the resistance and inductance of each of the coils were similar. Asingle resonant circuit was constructed using the four parallel coilsand non-magnetic tunable capacitors (Voltronics, Denville, N.J.) to tuneand match the circuit. The variable capacitors were located directlybeneath the sample region. With all four coils in parallel the circuithas a tuning range of ˜2 MHz centered around 300 MHz and a resonant Q ofabout 60. The coil housing was mounted atop a narrow-bore (about 39 mmdiameter) probe body and used a semi-rigid copper coaxial line toconnect the resonant circuit at the top of the probe to a BNC connectorat the base. To allow flow introduction of samples, teflon tubes (about2.0 mm outer diameter, Small Parts Inc., Miami Shores, Fla.) wereconnected to the capillaries using polyolefin heat-shrink (Small PartsInc., Miami Shores, Fla.) and sealed with Torr-Seal (Varian Associates,Palo Alto, Calif.). Samples were loaded using a syringe.

The probe was centered in an NMR magnet by loading four H₂O/D₂O samplesand adjusting the linear gradient (Z1 shim) to separate the peaks fromthe individual coils. The center was chosen as the point at which thetop and bottom coils were shifted in frequency by an equal and oppositeamount. In order to identify the NMR spectrum as originating from aparticular sample volume, the external reference procedure was utilizedfor this experiment. Gradient field adjustments were made by changing aZ1 shim power supply control for a Z-directional gradient coil of theNMR magnet. The nongraded composite spectrum of the samples was acquiredwith the Z1 shim set at its optimum value, and a second spectrum wassubsequently acquired with Z1 set to a value which results in apredetermined shift for each sample coil. This shift was calibratedbeforehand and remained relatively constant for a given coilconfiguration.

Spectra were collected using a Varian Unity-Plus spectrometer operatingat 300 MHz for ¹H. Typically, one or four transients (with Cyclopsreceiver phase cycling and a recycle delay of 5 s) were accumulated foreach experiment. A composite ¹H NMR spectrum is shown in FIG. 16 for 500mM samples of H₂O (δ=4.7 ppm), methanol (3.2 and 4.7 ppm), acetonitrile(1.9 ppm), and t-butanol (1.1 and 4.7 ppm) in D₂O that had been loadedinto separate coils of the flowing sample four-coil probe device. Theline widths (FWHM) are, respectively, 3.1 Hz, 2.8 Hz, 3.6 Hz, and 3.4Hz. Typical 90° pulse times were 6 μs using a transmitter power ofroughly 1 W. The measured mass sensitivity, S_(m), (defined as S/N perμmol of analyte) was 4200 for the t-butanol peak after 1 acquisition,using an apodization of 3 Hz (S_(m)=2700 for an apodization of 1 Hz).Pulse calibration data showed that the four coils have similar 90° pulselengths and RF field homogeneity. It should be appreciated that the FIG.12 and FIG. 16 spectra were based on the same samples. The peaks of theFIG. 16 spectrum differed slightly from the peaks of the FIG. 12spectrum due to typical variations in equipment and field settings.

As discussed above, upon the application of a linear field gradient, theNMR spectrum of each analyte is shifted by a value dependent on itslocation in a particular sample coil. The strength of the gradient wasadjusted by changing the value of the first-order axial (Z1) shimsetting by increments of 1000 (out of a possible±32,000), correspondingto an increase of the field gradient strength of roughly 12 mG cm⁻¹ perincrement. FIG. 17 shows a series of ¹H spectra obtained with the coilsloaded with same four samples as for FIG. 16, but now recorded withdifferent values of the applied field gradient. In FIG. 17, spectrum 400a is the same as the spectrum of FIG. 16, with the shim value set to 0mG cm⁻¹. Spectrum 400 b of FIG. 17 is provided with the shim value setto generate a gradient of 12 mG cm⁻¹. Also in FIG. 17, spectrum 400 c isprovided with the shim value set to generate a gradient of 24 mG cm⁻¹,and spectrum 400 d is provided with the shim value set to generate agradient of 48 mG cm⁻¹.

Each peak is shifted to a position given by δ^(i) _(shift)=δ^(i)_(iso)+γ×G_(z)×z_(i); where z_(i) is the vertical position of the i-thcoil, γ is the gyromagnetic ratio, and G_(z) is the strength of thegradient. G_(z) can be expressed as k×Z1; where k is a gradient strengthcalibration constant which depends on the shim hardware and isrelatively constant for all four coils. It can be seen that some of thepeaks (water and methanol) move downfield, and some more upfield(acetonitrile and t-butanol) depending on the sample location relativeto the gradient. Note that the overlapping OH peaks appearing at 4.7 ppmin spectrum 8 a are resolved by applying the gradient. For thisexperimental set-up, k was determined to be about 0.012 mG cm⁻¹ Z1 ⁻¹.Along with the frequency shift, there is a concomitant increase in theline width since missetting Z1 from its optimal value introduces aninhomogeneous field across each sample. The magnitude of this broadeningis on the order of G_(z)×d; where d is the diameter of the capillaries.In these experiments, the broadening is roughly 25% of the observedshift introduced by the applied gradient and is in agreement with thephysical dimensions and separation of the capillary samples.

The spectra were phased using Varian's VNMR software and converted to atext file using a translation program written in C. The spectral textfile was then transferred to a PC form of processor 119 for analysis. Aprogram was written to perform each of the first two analysis methodsdescribed above, and used as input: the number of coils, the frequencyshift (Δδ^(i)) for each coil arising from the applied field gradient,the digital resolution, and a threshold value for peak-picking, as wellas the two spectra. Use of procedure 310 resulted in correct peakassignments for all the peaks from the four samples.

The flow chart of FIG. 18 depicts procedure 410 of yet another techniquefor differentiating the combined spectrum of different samples 134 inprobe device 120. As in the case of earlier described proceduresassociated with the execution of stage 230 of process 210, processor 119may be partially or completely programmed to execute procedure 410.Procedure 410 begins with stage 412 which sets the coil counter CC to 1(CC=1). Procedure 410 then continues with a processing loop startingwith stage 414. In stage 414, the S_(G) spectrum is back-shifted by thepreviously determined frequency shift for the currently indexed coil CCas determined for the magnetic field gradient applied. This back-shiftedspectrum is designated as spectrum S_(B). In stage 416, the back-shiftedspectrum S_(B) is subtracted from the original spectrum S₀ to provide adifference spectrum S_(D). Generally, the difference spectrum S_(D)corresponds to spectral information from samples 134 of all coils 130except the currently indexed coil CC of interest. In stage 418, negativepeaks with an absolute magnitude exceeding an empirically determinedthreshold, such as the baseline average, are set to zero to provide amask spectrum S_(M). Proceeding with stage 420, the mask spectrum S_(M)is applied to the baseline spectrum S₀ to cancel all peaks contributedby samples 134 except for the sample 134 of interest in the coil 130indexed as coil CC.

Procedure 410 continues with conditional 422. Conditional 422 testswhether the currently indexed coil CC is the last coil. If the test ofconditional 422 is negative, then procedure 410 continues with stage 424to increment the coil counter CC to the next coil. Procedure 410 thenloops back to stage 414 to re-execute stages 414, 416, 418, and 420 withthe new coil index CC. Once the processing loop has been executed foreach coil 130, then the test of conditional 422 is true.Correspondingly, after procedure 410 has differentiated each samplespectrum from the S₀ and S_(G) spectra, it halts.

An experimental example of procedure 410 is depicted in connection withFIGS. 19-23 for the separation of the spectrum for the t-butanol sample.For this example, the S₀ spectrum is the spectrum of FIG. 16 with a Z1shim of about 0 mG/cm (also spectrum 400 a of FIG. 17) and the S_(G)spectrum is spectrum 400 d of FIG. 17. For this gradient component, theshifts Δδ^(i) for the four coils were 40.0 Hz (methanol), −10.0 Hz(acetonitrile), −61.1 Hz (t-butanol), and 91.0 Hz (water). Theexperimental set-up for the experimental example of procedure 410 isotherwise as provided for the experimental example described inconnection with procedure 310. In correspondence with stage 414 ofprocedure 410, the program for processor 119 starts by shifting eachpoint “n” in the shifted spectrum S_(G) to δ_(n) ^(i)−Δδ^(i); (whereδ_(n) ^(i) is its original position in the second spectrum, and Δδ^(i)is the expected shift for the i-th coil). FIG. 19 is a diagram of theresulting back-shifted spectrum S_(B) obtained by shifting spectrum 400d by the predetermined frequency shift. FIG. 20 is a diagram of adifferent spectrum S_(D) obtained by subtracting the back-shiftedspectrum S_(B) of FIG. 19 form spectrum 400 a in accordance with stage416 of procedure 410. As can be observed, two of the peaks for thesample of interest (t-butanol) cancel one another while the peaks fromthe other coils are inverted. FIG. 21 is the mask spectrum S_(M)obtained by setting all negative peaks appearing in the differencespectrum S_(D) of FIG. 20 with an absolute magnitude greater than orequal to the baseline average to zero per stage 418 of procedure 410.FIG. 22 is a diagram of the differentiated spectrum obtained bysubtracting the mask spectrum S_(M) of FIG. 21 from the baselinespectrum 400 a as described in connection with stage 420 of procedure410. In this case, the resulting spectrum contains only peaks due to thet-butanol sample.

Referring to FIG. 23, the differentiated sample-specific spectradetermined from spectra 400 a and 400 d with procedure 410 areillustrated. In FIG. 23, spectrum 430 a is the same as the compositespectrum 400 a of FIG. 17 and the spectrum shown in FIG. 16. Spectrum430 b of FIG. 23 is for the acetonitrile sample (peak at 1.9 ppm).Spectrum 430 c of FIG. 23 is for the water sample water (peak at 4.7ppm). Spectrum 430 d of FIG. 23 is for the t-butanol sample (peaks at4.7 ppm and 1.1 ppm). Spectrum 430 e of FIG. 23 is for the methanolsample (peaks at 4.7 ppm and 3.2 ppm).

Still another technique to differentiate a composite NMR spectrum ofdifferent samples is to apply reference deconvolution. Referencedeconvolution is performed by multiplying the experimental time-domaindata (Free Induction Decay data or “FID”) by the complex ratio “R” of anideal FID and a reference FID. This procedure can also be incorporatedinto programming for processor 119. The reference FID is constructedfrom the experimental spectrum by zeroing all parts of the experimentalspectrum except those containing the well-resolved reference signal andtaking its inverse Fourier transform. The ideal FID is similarlygenerated by placing a single point (delta function) at the peak of thereference signal and zeroing the rest of the spectrum, followed byinverse Fourier transformation. The corrected experimental FID iscalculated by multiplying the experimental FID by R, and Fouriertransformation yields the corrected FID.

Reference deconvolution is easily incorporated into procedure 410. Inone implementation, both spectra S₀ and S_(G) were deconvolved to thesame line width prior to performance of procedure 410. Because ofpossible nonuniform line-broadening across the detection coils 130, useof a standard for each coil is advisable. Typically, for desiredperformance, such a standard should include reference peak(s) from eachcoil that are well separated from the rest of the composite spectrum andmay be run with the samples or separately.

FIG. 24 demonstrates the effect of applying a reference deconvolution toboth the S₀ and S_(G) spectra prior to procedure 410 execution for amethanol peak. The original peak in the S₀ spectrum 500 a has a muchnarrower line width than the same peak in the S_(G) spectrum 500 c withthe applied gradient. After both S₀ and S_(G) were deconvoluted toLorenztian line shapes with 2 Hz linewidths, as shown in spectra 500 band 500 d, respectively, it is apparent that their intensities matchmuch better. As can be seen from these results, reference deconvolutionfacilitates conversion of the broad asymmetric peaks in S_(G) to peakswith line shapes that more closely resemble those in S₀. The inset 502of FIG. 24 compares the results of applying procedure 410 to themethanol portion of the data set with and without referencedeconvolution.

To improve the separation of more complex sample spectra from acomposite spectrum representative of multiple samples, multidimensionalNMR techniques may be incorporated into stage 230 of process 210 (FIG.9) in other embodiments of the present invention, including, but notlimited to two-dimensional (2D) Correlated Spectroscopy (COSY). Byspreading out the resonances into two or more dimensions, highly denseone dimensional (1D) spectra can be considerably simplified. Incorrespondence with one experimental example, FIG. 25 shows the COSYspectrum of 0.50 M 1-propanol, dichloroacetic acid, ethanol, andacetaldehyde in D₂O each loaded into a different sample coil. Thiscomposite spectrum was acquired with a substantially homogeneousmagnetic field in correspondence with stage 214 of process 210. Thespectrum shows a number of well resolved peaks, including five crosspeaks. Several of the OH peaks are missing because they have exchangedwith the deuterated solvent.

In FIG. 26, an overlap of unshifted and gradient shifted spectra in theregion of 0-7 ppm for both dimensions using the same samples is shown.Labeling the coils sequentially from top (coil 1) to bottom (coil 4),coil 1 resulted in peaks that are highly shifted upfield (by about 57Hz). The coil 2 peaks are also shifted upfield (by about 27 Hz). Thecoil 3 peaks are shifted somewhat downfield (by about 12 Hz), while thecoil 4 peaks are shifted further downfield (by about 51 Hz). For thiscase, the analytes can be assigned to their corresponding coils directlyfrom the two 2D spectra superimposed in FIG. 25. For example, theanalyte in coil 1 contains three diagonal peaks at 1.0, 1.7, and 3.7 ppmalong with two corresponding cross peaks, and is identified as1-propanol. Note that the OH resonance and corresponding cross peaks aremissing due to its exchange with the D₂O solvent. The coil 2 peak at 6.5ppm contains no cross peaks, and are identified as dichioroacetic acid.From the coil 3 peaks appearing at 1.3 and 3.8 ppm and the correspondingcross peaks, this sample can be identified as containing ethanol. Thecoil 4 sample includes diagonal peaks at 2.4 and 9.8 ppm, as well as twocorresponding cross peaks that were identified as acetaldehyde. Inaddition, there are two other diagonal peaks at 1.5 and 5.4 ppm, andtheir cross peaks. Since these peaks are shifted by the same amount andin the same direction as the acetaldehyde peaks, this species mustoriginate from the same coil. The chemical shift of these peaks areidentical to the literature values of 2,4,6-trimethyl-s-trioxane,(CH(CH₃)O)₃, which is a polymer of acetaldehyde, and which constitutesan impurity. The final assignments are: propanol in coil 1;dichloroacetic acid in coil 2; ethanol in coil 3; and acetaldehyde withan impurity in coil 4.

It is also possible to use the multiplication algorithm to generateseparate 2D sub-spectra, as was the case for the spectra described inconnection with FIG. 14. In accordance with this procedure, spectrumS_(G) is frequency shifted for each coil and multiplied with theoriginal spectrum S₀ according to equation (3) as follows:

S ^(i)(ω₁,ω₂)=S ₀(ω₁,ω₂)×S ^(i) _(G)(ω₁+Δ_(i),ω₂+Δ_(i))  (3)

Due to the high density of peaks along the diagonal in the 2D COSYspectra, there may be significant overlapping of peaks between theback-shifted and the original spectra that gives rise to spurious peaksalong the sub-spectra diagonal. This high level of congestion along thediagonal in 2D spectra would make it difficult to rely on these peaksfor coil assignment. However, by instead using the cross peaks,advantage may be taken of the generally higher resolution inherent inthis type of multidimensional NMR. The diagonal peaks of S^(i) aresuppressed in accordance with equation (4) as follows:

S ^(i′)(ω±δ,ω±δ)=0  (4)

This technique sets all the peaks within the bandwidth, δ, of thediagonal to zero, which does not affect off-diagonal peaks as long as δis smaller than the smallest J-coupling observed in the spectrum. Nowthe spectrum consists of only the cross peaks from samples in the ithcoil. The results of this procedure are shown in FIGS. 27 and 28 for twosub-spectra corresponding to 1-propanol and ethanol, respectively.

One alternative method is to apply pulsed field gradients during aportion of the NMR experiment. For example, a small pulsed fieldgradient applied during the acquisition time (t2) of the COSY experimentwill result in a 2D spectrum in which the separate samples are shiftedin frequency along a single frequency axis, in this case F2.Alternatively, large pulsed field gradients can be used to generatesubspectra in a different manner. A large pulse field gradient willshift the sample spectra into completely different frequency ranges suchthat individual spectra corresponding to the individual samples can beobtained directly. This is accomplished by the application of a largepulsed field gradient in conjunction with frequency selective RFexcitation pulse that excites only a single sample by taking advantageof the unique frequency shift provided by the applied field gradient.

The flow chart of FIG. 29 depicts process 610 for determiningsample-specific spectra from a composite spectrum. In stage 612 ofprocess 610 samples 134 are loaded into samples holders 132 disposedwithin coils 130 and sample counter SC is set to 1 (SC=1) with probedevice 120 being disposed within sample space 115.

From stage 616 processing loop 617 is entered beginning with stage 618.In stage 618 a magnetic field with gradient component G_(Z) is applied.Typically, the magnitude of this gradient is sufficient to provide asample region frequency shift large enough to separate each sample intoa different frequency range. In one nonlimiting example, for a spectralbandwidth of 6000 kHz (10 ppm for a 600 MHz magnet) and a coilseparation of about 5 mm, a field gradient of 3 gauss/centimeter wouldbe sufficient to provide the desired separation. Accordingly, in stage620, an excitation signal of frequency for f_(SC=1) is provided toexcite the indexed sample SC=1. The corresponding spectral data isgathered as spectrum S_(SC=1) in stage 622. Referring additionally tothe diagram of FIG. 30, the “with gradient” row provides a schematicillustration of sample spectrum S_(SC=1) at excitation frequencyf_(SC=1). For comparison, the “no gradient” row includes a schematic ofthe composite spectrum S₀ for excitation frequency f₀ that would beobtained for a unshifted magnetic field B₀.

Process 610 proceeds to conditional 624 to determine if the last samplehas been interrogated. If the last sample has not been interrogated,processing loop 617 continues with stage 626. In stage 626 the samplecounter SC is incremented to point to the next sample (SC=2), thenprocess 610 continues with stage 618, 620, and 622. During thisexecution of stage 620, a different excitation frequency f_(SC=2) isapplied corresponding to the new sample SC=2. The second entry on the“with gradient” row of FIG. 30 provides a schematic illustration ofsample spectrum S_(SC=2) at excitation frequency f_(SC=2).

The execution of stages 618, 620, and 622 continues for each remainingsample (SC=3 to SC=LAST), where LAST=4 for the probe device 120embodiment. Once execution of loop 617 for the last sample 134 iscomplete, process 610 terminates with from the affirmative branch ofconditional 624. Because each execution of processing loop 617 excitesonly a single sample, it can be repeated at a rate faster (typically oneor more per second) than the relaxation time (typically 10 seconds),permitting a fast sequence of data collection for each sample group. Asthe diagram of FIG. 30 illustrates, the application of a gradientsufficient to separate each sample in terms of frequency provides asurprising way to decompose a composite spectrum (i.e. S₀) of multiplesamples. In one embodiment, one or more gradient field coils dedicatedto spectra separation are included in system 210 (not shown) to providedesired field gradient qualities for process 610. In other embodiments,different gradient field sources and/or control arrangements may beutilized.

Processes 210, 610 and procedures 310, 410, 510 described in connectionwith system 110 may be combined, substituted, rearranged, reordered,deleted and altered as would occur to those skilled in the art for anapplication of interest. Moreover, for process 210, the first magneticfield B₀ provided in stage 214 need not be substantially homogeneous.Instead, in other embodiments, B₀ may be a known gradient differencerelative to the field applied in stage 218 that is not homogeneousthrough sample space 115. Likewise, system 110 and one or more of theseprocesses and procedures may be combined with one or more isolatedtuning network/coil combinations described for system 10. In still otherembodiments, the sampling parallelism of system may be generallyincreased as a function of the size of the homogeneous region of the NMRmagnet. In one example, for a 7.05 T wide-bore magnet, this regionextends over 20 mm. In an example having microcoils capable of acquiringhigh-resolution data with outer diameters on the order of 350micrometers (μm), and accounting for any broadening upon application ofthe gradient, the minimum coil spacing should be on the order of thecoil diameter. These parameters allow for at least 10 microcoils to belocated in the 20 mm region of the magnet. Such a probe would providefor a corresponding 10-fold reduction in throughput, which inconjunction with flow-through samples, would represent a significantadvance in high-throughput screening compared to conventional singlesample NMR techniques.

FIG. 31 provides a schematic view of NMR system 710 of anotherembodiment of the present invention. System 710 includes NMRinstrumentation 711 with RF excitation transmitter (TXR) 712 and NMRreceiver (RXR) 718. Receiver 718 is coupled to probe network 731 bytransmission line 736. Network 731 includes coils 730 a, 730 b eachdisposed about sample holder 732 a, 732 b, respectively. Each sampleholder 732 a, 732 b is depicted as the “flow-through” type previouslydescribed and is configured to receive a corresponding sample 734 a, 734b. Coils 730 a, 730 b are each of a solenoid configuration wound about acorresponding centerline axis 733 a, 733 b. Coils 730 a, 730 b areoriented so that centerline axes 733 a, 733 b are approximatelyparallel.

RF transmitter 712 of instrumentation 711 is coupled to a separateexcitation coil 740 to provide an RF stimulus signal to samples 734 a,734 b. Coil 740 is wound and centered relative to an axis that isgenerally perpendicular to the view plane of FIG. 31 and axes 733 a, 733b. Probe network 731 and excitation coil 740 are arranged to be placedin a magnetic field suitable to perform NMR analysis. For the describedorientation of coils 730 a, 730 b, 740; samples 734 a, 734 b will beexcited generally in-phase with one another by a suitable RF signal fromcoil 740, as designated by the common direction of the arrows in sampleholders 732 a, 732 b.

It should be understood that coils 730 a, 730 b are electricallyconnected to each other in series. Moreover, coil 730 a is wired fromleft to right and coil 730 b is oppositely wired from right to left.While samples 734 a, 734 b in coils 730 a, 730 b are excited in the samephase with coil 740, the opposite winding pathways of coils 730 a, 730 bprovide a phase offset in relation to corresponding sample responsesignals provided by coils 730 a, 730 b. When coils 730 a, 730 aresubstantially similar except for the opposite winding pathwaydirections, approximately a 180 degree phase difference results,providing at least partial cancellation of any peak common to bothsamples 734 a, 734 b.

Referring additionally to the diagram of FIG. 32, selected operationaspects of system 710 are further described. In FIG. 32 spectralresponse S₁ of coil 730 a is illustrated that corresponds to theresponse of an analyte and solvent mixture comprising sample 734 a. Alsoillustrated is spectral response S₂ of coil 730 b that corresponds tosample 734 b consisting of only the solvent of the mixture of sample 734a. Because of the opposite phase orientation of coils 730 a, 730 b,spectral response S₂ is illustrated as a negative peak. Accordingly, byoperation of the serial connection between coils 730 a, 730 b, spectralresponses S₁ and S₂ combine to substantially cancel the opposingresponses of the solvent, leaving only the analyte in the observedspectrum OS as illustrated in FIG. 32.

Referring to FIG. 33, system 810 of another embodiment of the presentinvention is illustrated. System 810 includes a transmitting networkcomprised of NMR transmitter 812 to selectively provide an RF excitationsignal, a matching network 813, and a crossed diode pair DP1. System 810also includes probe network 831 arranged for placement in a magneticfield suitable to perform NMR analysis. A receiving network 841 ofsystem 810 includes crossed diode pair DP2 and transmission line 836that are electrically coupled to probe network 831. Receiving network841 also includes matching network and NMR receiver 818 that areelectrically coupled to transmission line 836 opposite probe network831.

Probe network 831 includes coils 830 a, 830 b generally arranged to havemutually parallel longitudinal centerline axes as described inconnection with coils 730 a, 730 b of system 710. Coils 830 a, 830 b areeach disposed about a corresponding sample holder 832 a, 832 bconfigured to receive a respective sample 834 a, 834 b. As in the caseof probe network 731, coils 830 a, 830 b are electrically connected inseries to one another with respect to receiving network 841 and alsohave opposite winding path directions as described for system 710.Accordingly, transmitting network 821 is operable to excite samples 834a, 834 b in parallel with an in-phase relationship as symbolized by thearrows in holders 832 a, 832 b that point in a common direction;however, receiving network 841 is operable to receive the correspondingresponse from coils 830 a, 830 b in series. When coils 830 a, 830 b aresubstantially similar except for the opposite winding pathwaydirections, approximately a 180 degree phase difference resultsproviding at least partial cancellation of any resonant frequency commonto both samples 834 a, 834 b.

As explained in connection with FIG. 32, samples 834 a, 834 b can beselected to reduce the resulting spectral response of constituentscommon to both samples. Occasionally, a solvent suitable to dissolve anNMR analyte of interest provides undesirable spectral contributions.Advantageously, the ability to attenuate or cancel the spectralcontribution of a substance common to both samples for the“antiparallel” coil configurations of systems 710, 810 provides a meansto suppress solvent response in a solvent/analyte mixture. As in thecase of the previously described systems 10, 110; systems 710, 810 maybe adapted to use coils of different geometries. Also, the arrangementsof systems 710, 810 may be combined with the probe circuitry of system10 and/or 110 as would occur to those skilled in the art.

FIG. 34 depicts NMR system 910 of still another embodiment of thepresent invention. System 910 includes NMR instrumentation 111 with NMRRF transmitter (TXR) 112, controller 113, and NMR receiver (RXR) 118 ofthe type previously described in connection with system 110 of FIG. 6.Likewise, system 910 includes processor 119 coupled to instrumentation111 in the manner previously described for system 110. Instrumentation111 is electrically coupled to probe device 920 by transmission line936. Probe device 920 is shown in partial section and is disposed insample space 115 of NMR magnetic field source 116 also previouslydescribed in connection with system 110. System 910 may further includesample control instrumentation of the type provided in system 110 (notshown).

Probe device includes housing 950 extending from base 960. Housing 950defines chamber 966 that contains several printed circuit substrates 921a, 921 b, 921 c (collectively designated circuit boards 921). Eachcircuit board 921 includes conductive material 922 electricallyconnected to ground. Conductive material 922 is arranged to providesuitable electromagnetic shielding between various components mounted oneach of circuit boards 921. In one nonlimiting example, the side of eachcircuit board 921 exposed to the components of another of the circuitboards 921 is clad with a copper plating that is grounded to serve asconductive material 922, and provide a corresponding ground plane.

Each circuit board 922 further includes an NMR detection coil 930 a, 930b, 930 c (collectively designated coils 930), respectively. Coils 930are each arranged to be disposed about a different sample submitted forNMR analysis analogous to the manner described for samples 134 of system110; however, coils 930 are each mounted to its respective circuit board922. Circuit boards 922 further include trimming/tuning components, afew of which are designated by reference numeral 935. Components 935have selected electrical connections with coils 930 and can beconfigured to adjust for small differences in coil inductance orresistance. Accordingly, components 935 are commonly in the form ofresistors and/or capacitors, but may alternatively or additionallyinclude other types as would occur to those skilled in the art.

Each of coils 930 can be of a solenoid, saddle, or such other coilgeometry as would occur to those skilled in the art. A longitudinalcenterline axis 933 a for detection coil 930 a is illustrated that isgenerally parallel to a longitudinal centerline axis 933 c illustratedfor detection coil 930 c. Axes 933 a, 933 c are also generally parallelto the view plane of FIG. 34. In contrast, detection coil 930 b has alongitudinal centerline axis that is generally perpendicular to the viewplane of FIG. 34 as represented by cross hairs designated with referencenumeral 933 b. Accordingly, axis 933 b crosses axes 933 a, 933 c atapproximately right angles (90 degrees). This 90 degree orientation ofone coil 930 to the next has been found to further reduce undesirableelectromagnetic coupling between adjacent coils. Probe device 920further includes tuning circuit 931 adjacent probe base 960 thatelectrically interconnects coils 930 and components 935 to transmissionline 936. Coils 930 a, 930 b, 930 c are each disposed about acorresponding sample holder 932, 932 b, 932 c of the flow-through type(collectively designated holders 932). Samples disposed within holders932 and coils 930 receive RF excitation signals from instrumentation111, and corresponding response signals are detected withinstrumentation 111 as previously described for system 110.

The arrangement of coils and circuitry in probe device 920 may includemore or fewer coils. Alternatively or additionally, dedicated RFexcitation coils may be employed in system 910 instead of exclusivelyrelying on coils 930 to perform excitation and detection. Furthermore,more than one coil 930 per circuit board 921 may be included and/or oneor more of coils 930 may be configured with a separate, isolated turningnetwork as in the case of system 10. Coils 930 may be configured likethe multicoil probe circuit of system 110, system 710 and/or system 810,just to name a few.

Many further embodiments of the present invention are envisioned. Forinstance, in alternative embodiments, intensity, line width, and/ormultiple information may be incorporated in the logic of the variousprocedures being used to discriminate sample-specific spectra from oneor more complicated, overlapping composite spectra. In still otherembodiments, data analysis may include linear prediction and digitalfiltering. Also, alternative embodiments may include fewer or morecoils; coils of different geometries; different combinations of systems10, 110, 710, 810, 910; different combinations of processes 210, 610and/or procedures 310, 410. In further embodiments, systems andprocesses of the present invention are adapted to interrogate samples ina solid phase and/or may not include a “flow-through” samplingarrangement. Also, standard sample spinning apparatus may beincorporated into the systems of the present invention using techniquesknown to those skilled in the art.

In general, the various embodiments of the present invention providecorresponding techniques to simply and cost-effectively increase NMRsample throughput. In embodiments including the investigation ofStructure Activity Relationships (SAR) by NMR, one is interested inidentifying only the molecules that interact strongly with largeproteins. These molecules typically have significantly shorterrelaxation times than other, non-interacting molecules. Therefore, byusing spectral editing methods to discriminate against thenon-interacting molecules, parallel sample coils of the presentinvention may be used to advantage. In other embodiments, probes areutilized for process monitoring and/or control. Furthermore, thedeviations of NMR spectrums from a known standard spectrum may bemonitored according to the present invention to identify potentialproblems. Moreover, the present invention includes embodiments havingmulti-coil probes for monitoring multiple reactions or processes inparallel.

Other embodiments of the present inventions include applications to abroad range of problems in analytical chemistry. For example, there is agrowing need for the rapid analysis of large numbers of compounds in thepharmaceutical industry to identify potential drug candidates. In thisarea, Heteronuclear Multiple Quantum Coherence (HMQC) techniques can beapplied to investigate SAR. S. B. Shuker, P. J. Hajduk, R. P. Meadows,S. W. Fesik, Science 274 (1996) 1531; and P. J. Hajduk, E. T.Olejniczak, S. W. Fesik, Journal of the American Chemical Society 119(1997) 12257 are cited as sources of additional information concerningHMQC techniques. Parallel NMR analysis will be advantageous in such anapplication. In another embodiment, for combinatorial chemistry, wherelarge numbers of somewhat similar compounds are quickly synthesized, arapid NMR analytical method could be desirable. Also, embodiments of thepresent inventions include coupling the structural determinationcapabilities of NMR with chromatographic separation techniques such asLC-NMR and CE. Other variations and embodiments of the present inventioninclude utilizing the unique NMR probe designs of the present inventionwith microcoils, flow-through probes, nano-volume probes and solventsuppression pulse sequences—such embodiments may also includecombinatorial synthetic methods and methods to analyze large molecularlibraries.

It is contemplated that various operations, stages, conditionals,procedures, thresholds, routines, and processes described in connectionwith the present invention could be altered, rearranged, substituted,deleted, duplicated, combined, or added as would occur to those skilledin the art without departing from the spirit of the present invention.As used herein, it should be appreciated that: spectrum, spectra,variable, criterion, characteristic, comparison, quantity, amount,information, value, level, term, constant, flag, data, stage, record,threshold, and limit each generally correspond to one or more signalswithin processing equipment of the present invention.

All references to experiments and results are intended to beillustrative of the present invention and should not be consideredlimiting or restrictive with regard to the scope of the presentinvention. Further, any theory of operation, proof, or finding statedherein is meant to further enhance understanding of the presentinvention and is not intended to make the scope of the present inventiondependent upon such theory, proof, or finding. All publications,patents, and patent applications cited in this specification are hereinincorporated by reference as if each individual publication, patent, orpatent application were specifically and individually indicated to beincorporated by reference and set forth in its entirety herein.Documents to be incorporated by reference include, but are not limitedto: (1) U.S. Provisional Patent Application No. 60/121,869, filed Feb.26, 1999; (2) Hou, T.; MacNamara, E.; Raftery, D. Analytica Chimica Acta400 (1999) 297; (3) Fisher, G.; Williams, S.; Raftery, D. AnalyticaChimica Acta 397 (1999) 9-16 and (4) Fisher, G.; Pettuci, C.; Raftery D.Journal of Magnetic Resonance, 138 (1999) 160-163. While the inventionhas been illustrated and described in detail in the drawings andforegoing description, the same is to be considered as illustrative andnot restrictive in character, it being understood that only thepreferred embodiment has been shown and described and that all changes,equivalents, and modifications that come within the spirit of theinvention are desired to be protected.

What is claimed is:
 1. An NMR apparatus, comprising: an NMR transmitterfor transmitting one or more signals to a number of samples; a number ofsample holders each operable to receive a corresponding one of thesamples for NMR analysis; a plurality of detection coils each operableto detect a an individual response of a corresponding one of the samplesto one or more signals from said NMR transmitter; an adjustable magneticfield source proximate to a sample space arranged to receive said sampleholders, said magnetic field source being operable to selectivelyprovide: a first magnetic field to generate a first collective responseof the samples when received in said sample space and excited by saidone or more signals from said NMR transmitter, a second magnetic fieldhaving a gradient relative to said first magnetic field to generate asecond collective response of the samples when received in said samplespace and excited by said one or more signals from said NMR transmitter,said second collective response corresponding to a number of differentfrequency shifts relative to said first collective response, saiddifferent frequency shifts of said second collective response eachcorresponding to a different one of the samples; and a processoroperable to determine a number of spectral characterizations as afunction of said first collective sample response and said secondcollective sample response, the spectral characterizations each beingrepresentative of a different one of the samples.
 2. The apparatus ofclaim 1, wherein said first response corresponds to an unshiftedcomposite spectrum of the samples, said second response corresponds to ashifted composite spectrum of the samples, and said processor isoperable to establish a spectral mask for each of said detection coilsfrom said first response and said second response during determinationof the spectral characterizations.
 3. The apparatus of claim 1, whereinsaid processor is operable to perform reference deconvolution duringdetermination of the spectral characterizations.
 4. The apparatus ofclaim 1, wherein said processor is operable to perform multidimensionalNMR analysis.
 5. The apparatus of claim 1, wherein the first magneticfield is generally homogeneous in said sample space.
 6. The apparatus ofclaim 1, wherein said detection coils number at least four.
 7. Theapparatus of claim 1, further comprising an NMR receiver coupled to saiddetection coils and a control to selectively adjust the gradient.
 8. Theapparatus of claim 1, wherein said sample holders are each in fluidcommunication with sample instrumentation to perform capillaryelectrophoresis.
 9. The apparatus of claim 1, wherein said sampleholders are each mounted in a probe device sized for insertion in saidsample space, said probe device including a vessel to dispose asusceptibility fluid about said sample holders when received in saidsample space.
 10. An NMR method, comprising: operating an NMRspectroscopy system including a sample space, a transmitter, and aplurality of detection coils each disposed about a corresponding one ofa plurality of separated samples in the sample space; generating a firstmagnetic field in the sample space; exciting the samples substantiallysimultaneously with an RF signal during generation of the first magneticfield; detecting a first collective response of the samples as a resultof the first magnetic field and RF excitation; generating a secondmagnetic field in the sample space, the second magnetic field includinga gradient relative to the first magnetic field; exciting the samplessubstantially simultaneously with an RF signal during generation of thesecond magnetic field; detecting a second collective response from thesamples as a result of the second magnetic field and RF excitation, saidsecond collective response corresponding to a number of differentfrequency shifts relative to said first collective response, saiddifferent frequency shifts of said second collective response eachcorresponding to a different one of the samples; and determining anumber of spectral characterizations as a function of said firstcollective sample response and said second collective sample response,the spectral characterizations each being representative of a differentone of the samples.
 11. The method of claim 10, further comprisingperforming a multidimensional NMR analysis of the samples.
 12. Themethod of claim 10, wherein said determining includes establishing aspectral mask for each of the detection coils.
 13. The method of claim10, wherein said determining includes performing referencedeconvolution.
 14. The method of claim 10, wherein the first and secondsample responses each correspond to a composite spectrum.
 15. The methodof claim 10, wherein said detection coils number at least four and areeach of a microcoil variety with a diameter of less than about 1millimeter.
 16. The method of claim 10, wherein the detection coils areeach disposed about a respective one of a number of tubes, and the tubesare each arranged to receive the corresponding one of the samples. 17.An NMR apparatus, comprising: an NMR transmitter for transmitting one ormore signals to a number of samples, wherein the number of samples istwo or more; a number of sample holders each operable to receive acorresponding one of the samples for NMR analysis; a plurality ofdetection coils electrically connected in parallel, each detection coiloperable to detect a response of the corresponding one of the samples toone or more signals from said NMR transmitter; an adjustable magneticfield source proximate to a sample space arranged to receive said sampleholders, said magnetic field source being operable to selectivelyprovide: a first magnetic field which generates a response from at leasta first one of the samples when received in said sample space andexcited by an RF signal from said NMR transmitter, a second magneticfield having a gradient relative to said first magnetic field togenerate a response from at least a second one of the samples whenreceived in said sample space and excited by an RF signal from said NMRtransmitter, wherein the response from the second sample to said secondmagnetic field corresponds to a frequency shift relative to the responsefrom said second sample that would have occurred as a result of anexcitation during the generation of said first magnetic field; a commonreceiver connected to each of said detection coils; and a processorcoupled to an output of said common receiver and operable to determineseparate spectral characterizations for each response to the firstmagnetic field and each response to the second magnetic field.
 18. Theapparatus of claim 17, wherein the first magnetic field is generallyhomogeneous in said sample space.
 19. The apparatus of claim 17, whereinsaid detection coils are each of a microcoil variety with a diameter ofless than about one millimeter.
 20. An NMR method, comprising: operatingan NMR spectroscopy system including a sample space, a transmitter, anda plurality of detection coils each disposed about a corresponding oneof a plurality of separated samples in the sample space, wherein saiddetection coils are electrically connected in parallel and connected toa common receiver; generating a first magnetic field in the samplespace; applying a first excitation pulse to the sample space; detectinga response from at least a first one of the samples to the firstmagnetic field and said first excitation pulse; generating a secondmagnetic field in the sample space, the second magnetic field includinga gradient relative to the first magnetic field; applying a secondexcitation pulse to the sample space; detecting a response from at leasta second one of the samples to the second magnetic field and said secondexcitation pulse, wherein the response from the second sample to saidsecond magnetic field corresponds to a frequency shift relative to theresponse from said second sample that would have occurred as a result ofan excitation during the generation of said first magnetic field; anddetermining a plurality of NMR spectra from the responses, one NMRspectrum corresponding to the response from the first one of the samplesto the first magnetic field and first excitation pulse, another NMRspectrum corresponding to the response from the second one of thesamples to the second magnetic field and second excitation pulse. 21.The method of claim 20, wherein the first magnetic field is generallyhomogeneous in said sample space.
 22. The method of claim 20, whereinsaid detection coils are each of a microcoil variety with a diameter ofless than about one millimeter.